According to "Thrones" writer and producer Bryan Cogman's
book, "Inside HBO's Game of Thrones: Seasons 1 &
2," the prank occurred while shooting the eighth
episode of season one. In the real, aired
version, Snow sets a wight on fire at Castle
Black while protecting Lord Commander Mormont (James Cosmo).
But Harington didn't initially get that version of the
scene.

The producers sent Harington a script with a fake version
of the scene. In it, Jon Snow pulls burning drapes from a fire
and throws them at the wight. The fire, though, engulfs the both
of them. In the end, Mormont exclaims, "Gods, boy. I was
wrong about you. You’re a ranger, through and
through!"

The scene ends with: “Jon smiles, his teeth shining
brightly in his destroyed face. Mormont, sickened, has to look
away.”

Harington was prepared to shoot the fire scene, which Weiss
told him was the result of HBO being "worried the Jon Snow
storyline was too Harry Potter, and they wanted to do something
to make it darker."